Small particles of nickel sulfide (NiS) usually known as nickel sulfide "stones" sometimes occur in glass and result in severe degradation of the glass quality- Nickel sulfide stones are usually too small to be seen by the unaided eye and are very difficult to be detected by optical inspection means. Nickel sulfide stones as small as 40 microns may cause spontaneous breakage in tempered glass parts. This breakage is related, in part, to the slow phase change in the nickel sulfide stones which create localized stresses. Increased temperature may accelerate the phase change. In addition, there is a large difference between the coefficient of thermal expansion of nickel sulfide stones and that of glass. As a result, change in temperature of a glass product, such as a glass sheet installed in a building or a vehicle, that includes a nickel sulfide stone may further cause intense localized stresses in the vicinity of the stone which may be of sufficient magnitude to spontaneously fracture the sheet. Because it is difficult to detect the presence of nickel sulfide stones in glass, and because their effects may not be exhibited until long after the glass sheets have been installed, prevention of nickel sulfide stones is an important objective for glass makers.
The most straight forward approach to avoiding nickel sulfide stones is to prevent any source of nickel from entering the glass melting furnace; however trace amounts of nickel may appear as naturally occurring impurities in the raw materials used for making glass. Also, the common presence of nickel in stainless steel alloys used in equipment associated with the raw materials mining and handling and other machinery associated with a glass melting operation may lead to the inadvertent introduction of small amounts of nickel into the glass melting furnace, even when strenuous efforts are made to avoid its deliberate introduction. In addition, in some glass compositions, quantities of nickel are added to the batch as a colorant. Nickel may also be found in glass cullet used to make the glass.
It would be desirable if formation of nickel sulfide stones could be prevented in the output from a glass melting furnace in which nickel is present.
U.S. Pat. No. 4,919,698 reduces the occurrence of nickel sulfide stones in glass by electrically imposing oxidizing conditions near the bottom of a melting furnace and at least at a portion of the region between the inlet end and spring zone of the furnace to prevent the presence of reducing conditions that would create or sustain nickel sulfide stones. It should be appreciated that the primary source of nickel sulfide stones may not be from the bottom of the melter. Therefore, another method of stone reduction, i.e. one that addresses the reduction and/or prevention of nickel sulfide stones, regardless of the location of their formation within the melter, is required.